1. Field of the Invention
The present invention relates to a semiconductor device manufacturing method and, more particularly, to a method of removing a silicon oxide film on a silicon region or a silicide region.
2. Description of the Related Art
Along with an increase in packing density of a silicon device, cleaning in accordance with a dry process, e.g., dry etching, that performs processing in a vacuum is attracting attention to replace cleaning of the surface of a silicon substrate in accordance with a wet process that performs processing in a solution, e.g., dilute hydrofluoric acid processing. According to the wet process, the substrate must be exposed to the atmosphere after processing, causing problems such as microparticle contamination, organic compound contamination, reoxidation of the substrate surface, and the like. In contrast to this, according to the dry process, since processing can be easily performed in a high vacuum continuously, the clean surface can be maintained, thereby solving the problems of the wet process.
For example, in a NAND type EEPROM, an increase in reliability of a tunnel oxide film is strongly demanded. In this case, in the process including pre-processing of the substrate, formation of the tunnel oxide film, and formation of poly-Si, cleaning of the substrate surface before formation of the oxide film is important, and an increase in reliability in accordance with dry pre-processing is expected.
As a method of removing a silicon oxide film at room temperature, HF-Vapor (HF/H.sub.2 O vapor) process is conventionally reported. According to this method, the oxide film is removed by, e.g., the following process. First, an 800-nm thick field oxide film is formed by thermal oxidation on the surface of an n-type silicon substrate having a (100) plane as a major surface. To remove the organic contamination on the surface of the silicon substrate, processing with a mixed solution of sulfuric acid and hydrogen peroxide (to be referred to as SH processing hereinafter) is performed. As a chemical oxide is formed on the surface of the silicon substrate by the SH processing, it is removed by dilute hydrofluoric acid processing.
Subsequently, in order to remove metal contamination on the exposed surface of the silicon substrate, processing with a mixed solution of hydrochloric acid and hydrogen peroxide (to be referred to as SC2 processing hereinafter) is performed. By the SC2 processing, the surface of the silicon substrate is covered with a chemical oxide again.
After these processing operations employing solutions, the substrate is transferred into a chamber, and the chemical oxide is removed by HF-vapor (HF/H.sub.2 O vapor) processing. In the HF-vapor processing, N.sub.2 gas is supplied into an aqueous hydrofluoric acid buffer solution by bubbling, and the obtained N.sub.2 gas containing the HF/H.sub.2 O vapor is blown to the substrate maintained at room temperature.
Thereafter, a tunnel oxide film and a poly-Si layer are formed by continuous incineration oxidation using O.sub.2 /H.sub.2 and SiH.sub.4 gas, respectively, in the same chamber, and the silicon substrate is unloaded from the chamber. By this processing, the concentrations of impurities, e.g., organic matter, can be decreased in the interfaces of the poly-Si layer/SiO.sub.2 layer (tunnel oxide film)/Si substrate stacked structure.
However, recent intensive studies have made it apparent that the above method causes problems as follows. When the chemical oxide is removed in accordance with the HF-Vapor process, since an aqueous solution of HF is employed, particles, e.g., Si(OH).sub.x particles as the reaction product of the silicon oxide film and water, remain on the surface of the substrate, to cause morphological degradation during formation of the tunnel oxide film and poly-Si layer that follows. It has become apparent that, in order to solve this problem, rinsing with water shower must be performed as a particle removing step.
When the native oxide film is removed by the conventional wet process using dilute hydrofluoric acid or the HF-Vapor process described above, selectivity against the thermal oxide film cannot be obtained. Thus, if a different oxide film, e.g., a contact region, exists on the substrate, not only the native oxide film but also the other oxide films are entirely etched undesirably, thereby largely changing the contact diameter, the aspect ratio, and the like.
Furthermore, when the aqueous HF solution is exposed to a metal, HF forms active HF.sub.2.sup.- and F.sup.- on the surface of the metal. This brings about rapid fluoridization of the metal, thus corroding the metal. This phenomenon inflicts large damages to the interior of the chamber, the pipes, the exhaustion system, and the like.
As the partial pressure of the background water in the chamber is high, H.sub.2 O is mixed in the process gas in the following step, e.g., oxidation step. Then, the formed film contains oxygen and hydrogen to degrade the film quality. For this reason, in order to decrease the partial pressure of the water in the atmosphere, evacuation as a transient step to the following process must be performed for a long period of time, resulting in an increase in process time.
As described above, in the conventional oxide removing method, particles are formed on the surface of the silicon substrate to cause morphological degradation, and the evacuation time is prolonged to increase the process time.
As the operational speed and the integration degree of the LSIs increase, the packing density of the electrodes and wires increases. An increase in packing density of the electrodes and wires entails an increase in resistance. For this reason, metal silicides are currently widely used as the material of the electrodes and wires.
For example, conventional contact electrode formation using a metal silicide is performed in the following manner. First, an 800-nm thick field oxide film is formed by thermal oxidation on the surface of an n-type silicon substrate having a (100) plane as a major surface. BF.sub.2.sup.+ ions are implanted in the surface of the n-type silicon substrate in an element formation region surrounded by the field oxide film with an acceleration voltage of 35 eV and a dose of 5.times.10.sup.15 cm-.sup.2. The n-type silicon substrate is then heated to 1,000.degree. C. for 20 seconds in an N.sub.2 atmosphere, thereby forming a shallow p.sup.+ -type diffusion layer having a thickness of about 0.1 .mu.m.
A stacked layer consisting of a CVD-SiO.sub.2 film and a BPSG film is deposited as an insulating interlayer film on the entire surface to a thickness of 1.0 .mu.m. This stacked film is etched to form a contact hole on the p.sup.+ -type diffusion layer. Then, the n-type silicon substrate is washed with an aqueous dilute hydrofluoric acid solution to separate the native oxide film on the surface of the p.sup.+ -type diffusion layer.
The n-type silicon substrate is transferred into a vacuum unit. A stacked film consisting of a 30-nm thick Ti film and a 70-nm thick TiN film is formed on the silicon substrate by continuous sputtering. Silicidation in accordance with RTA (Rapid Thermal Anneal) processing is performed at 750.degree. C. for 30 seconds. Non-reacted portions of the Ti and TiN films are removed by processing with a mixed solution of sulfuric acid and hydrogen peroxide. As a result, a 60-nm thick TiSi.sub.2 layer is formed on only the p.sup.+ -type diffusion layer in self alignment. At this time, a SiO.sub.x layer having a thickness of about 4 nm is formed on the TiS.sub.2 layer. This is because a silicon oxide and the like can be easily formed on the surface of the metal silicide by water and oxygen in the atmosphere or by chemical processing with, e.g., a mixed solution of sulfuric acid and hydrogen peroxide.
The SiO.sub.x layer causes an increase in contact resistance and separation of a W film which is formed in the following step and thus must be removed. Therefore, the SiO.sub.x layer is removed by RIE (Reactive Ion Etching) using BCl.sub.3 gas.
Subsequently, the TiSi.sub.2 layer is heated to 350.degree. C., and monosilane (SiH.sub.4) and tungsten hexafluoride (WF.sub.6) are supplied to the n-type silicon substrate, thereby selectively forming a W layer on the TiSi.sub.2 layer. For example, the flow rate, total pressure, and supply time of monosilane and tungsten hexafluoride are in both cases 10 sccm, 0.15 Torr, and 60 seconds.
However, recent intensive studies have made it apparent that the above method has problems as follows. In removal of the SiO.sub.x layer 125 by RIE using BCl.sub.3 gas, if the aspect ratio of the contact hole is high, a microloading effect occurs in which ions collide against the side wall of the contact hole due to charge-up of the SiO.sub.2 film and the BPSG film. As a result, the orbits of the ions (B ions and Cl ions) serving as the etchants are bent, so that the amount of ions reaching the bottom portion of the contact hole decreases. As the SiO.sub.x layer undesirably remains on the edge region of the bottom surface of the contact hole, the W layer easily separates undesirably.
Furthermore, in above-described removal of the SiO.sub.x layer, as high-energy ions, e.g., B ions and Cl ions, in the BCl.sub.3 plasma are implanted in the TiSi.sub.2 layer, the TiSi.sub.2 layer is damaged, thereby increasing the contact resistance.
As described above, in the conventional contact electrode formation employing a silicide layer, as the oxides on the surface of the silicide layer are removed incompletely, the conductive film formed on the silicide film easily separates undesirably. In addition, the silicide layer is damaged during removal of the oxides, leading to an increase in contact resistance.